1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus that converts DC power to AC rectangular wave power and supplies the AC rectangular wave power to a discharge lamp. More specifically, the present invention relates to a discharge lamp lighting apparatus that can be utilized in an ideal manner to light a high-pressure discharge lamp such as a high-pressure mercury lamp or an ultra high-pressure mercury lamp with AC rectangular wave power.
2. Discussion of Background
It is known in the related art that a high-pressure discharge lamp can be lit with a relatively high degree of efficiency by supplying AC rectangular wave power with a low frequency of, for instance, approximately 50 to 500 Hz.
A discharge lamp lighting apparatus that lights a discharge lamp with AC rectangular wave power normally rectifies a commercial alternating current to convert it to direct current, and executes power control on the direct current by using a converter constituted with a step-down chopper circuit, and converts the power to a low-frequency AC rectangular wave current/voltage through an inverter constituted of a bridge circuit achieved by combining two or four semiconductor switch elements, and supplies the AC rectangular wave current/voltage to the discharge lamp.
JP Patent Publication No. 1991-116693A discloses a discharge lamp lighting apparatus that lights a discharge lamp with such AC rectangular wave power. The discharge lamp lighting apparatus disclosed in this patent publication includes a chopper circuit connected to a DC source, which operates with a high frequency, a bridge inverter circuit that is connected to the chopper circuit and is constituted of a switch element which operates at a low frequency and a load circuit that includes a discharge lamp connected to the output side of the bridge inverter circuit via a pulse transformer.
The pulse transformer is constituted as a closed magnetic circuit in order to minimize the magnetic flux leak. However, a pulse transformer constituted as a closed magnetic circuit poses a problem in that when the rectangular wave current flowing through the serial circuit constituted with the discharge lamp and the primary winding of the pulse transformer is inverted, the magnetic energy generated at the core of the pulse transformer changes drastically to induce a beat at the area where the core is joined.
Accordingly, the discharge lamp lighting apparatus disclosed in this patent publication implements control so as to reduce the current supplied by the chopper circuit in synchronization with the timing with which the switch element at the bridge inverter circuit is turned on/off in order to minimize the beat occurring at the pulse transformer.
However, there is another issue that must be addressed in addition to the occurrence of buzz in a discharge lamp lighting apparatus that lights a discharge lamp with AC rectangular wave power. Namely, a vibration attributable to the impedance characteristics of the circuit of the discharge lamp lighting apparatus and the impedance characteristics of the lamp itself may occur when the AC rectangular wave voltage/current is inverted to result in an occurrence of an overshoot. Such an occurrence of an overshoot leads to various problems with regard to the discharge lamp.
The following is an explanation of a state in which an overshoot occurs, given in reference to the drawings. FIG. 13 shows the waveforms of the output voltage from the converter, the output current from the inverter and bridge signals at the inverter, detected in the discharge lamp lighting apparatus which lights a discharge lamp with AC rectangular wave power. FIG. 14 shows a partial enlargement of the waveform diagram in FIG. 13. The output voltage/current from the converter, which is a controlled DC voltage/current, is converted to an AC rectangular wave voltage/current at the bridge inverter connected at a rear stage of the converter.
For this reason, while the output voltage from the converter and the output current from the inverter are individually controlled to sustain the voltage level and the current level needed by the lamp load until immediately before polarity inversion time points at which the ON/OFF states of the bridge signals 1 and 2 are switched over, vibration manifests as the polarity inversion occurs, as shown in FIG. 13.
To explain this point in further detail, an inverter is normally constituted of a bridge circuit by using semiconductor switch elements. In order to prevent shorting, the semiconductor switch elements in the bridge circuit are not allowed to enter an ON state simultaneously by implementing ON/OFF control on the semiconductor switch elements with dead time allowed to elapse at the time of a polarity inversion.
As shown in FIG. 14, the semiconductor switch elements are all set in an OFF state during the dead time period td, and thus, the energy communicated from the converter cannot reach the load, i.e., the lamp, thereby inducing a rise in the output voltage from the converter. In addition, the inductance component present in the circuit of the discharge lamp lighting apparatus induces a commutation of the current and the current flows from the discharge lamp to the converter, thereby causing a rise in the output voltage from the converter.
Following the dead time period td, the semiconductor switch element in the bridge circuit enters an ON state to allow the output voltage from the converter to be applied to the discharge lamp. Since the output voltage from the converter at this time is higher, the voltage/current supplied to the discharge lamp achieve larger values compared to the voltage/current values before the inversion, thereby causing vibration and overshoot.
The levels of the current/voltage being supplied to the lamp when such an overshoot occurs are excessively high for the discharge lamp. The electrode at the discharge lamp becomes damaged every time the state of excess current/voltage occurs as the polarity of the AC rectangular wave voltage/current is inverted, and with the electrode damaged in this manner constantly over time, the service life of the discharge lamp is reduced.
The extent of overshoot may be reduced by increasing the capacity of the output capacitor in the converter. In such a case, while the extent of increase in the output voltage from the converter can be minimized, the vibration cycle is lengthened to result in an increase in the length of time to elapse before the vibration becomes settled. If there is any residual vibration in the voltage/current supplied to the discharge lamp, problems arise in that the vibration in the light output from the discharge lamp manifests as flickering, in that the discharge lamp fails to startup fully or in that there is an increase in the rush current (shorting current) flowing to the discharge lamp as the AC rectangular wave voltage/current becomes inverted.
It is believed that a rise in the rush current (shorting current) flowing to the discharge lamp when the polarity of the AC rectangular wave voltage/current is inverted causes wear in the electrode of the discharge lamp, which will result in a reduced service life of the discharge lamp.
For this reason, it is necessary to ensure that the discharge lamp is lit in a desirable manner by adjusting the waveform of the voltage/current supplied to the discharge lamp when the polarity of the AC rectangular wave voltage/current becomes inverted and thus minimizing the extent of overshoot.
In addition, since the overshoot manifests to a great extent when the level of the current supplied to the discharge lamp is high, manifests to a small extent when the level of the current supplied to the discharge lamp is low and also manifests to fluctuating extent depending upon the accumulated lengths of time over which individual discharge lamps have been in an ON state, a discharge lamp lighting apparatus that allows the extent to which overshoot is reduced to be controlled is needed.